212 ASPHALT. 



condition of climate, traffic and grade; and in amount to suit 

 degree of traffic it will be called upon to sustain, and the mineral 

 aggregate available or required for the type of pavement to be 

 laid. 



Combination. 



The cold sand is deposited in the predetermined proportions 

 at the foot of the cold -material elevator of the paving plant by 

 which it is conveyed through a chute into the sand drier, a revolv- 

 ing drum heated underneath by a coal or oil fire. This drum is 

 inclined at a slight angle towards the back end of the plant. It 

 is provided with staggered nights of angle irons rivetted longitudin- 

 ally on the interior of the drum. This arrangement lifts the sand 

 as the drum revolves until the inclination is so great that the sand 

 slides off, and falling through the hot gases now returning through 

 the drum is deposited a little in advance of its starting point ; and 

 so it eventually reaches the end wheie it falls through a chute into 

 a boot at the foot of the hot sand elevator. From here it is lifted 

 vertically by the bucket elevator and dumped through another 

 chute into a revolving screen, half of which is made of J inch wire 

 mesh, and the other half of a 1 inch wire mesh. Here it is screened 

 into sand and rock respectively passing and retained on the ^ inch 

 screen which falls into bins underneath. The rock rejected by the 

 1 inch mesh screen passes away from the plant, or if desired it 

 may all be received into the rock bin. From the bottom of the bin 

 the hot sand at about 350° F. passes through a swinging gate into 

 the sand box supported on the platform of a triple beam scales 

 whereby the correct weight is obtained. The dust is added cold 

 in predetermined amount to the hot sand, and the total aggregate 

 is passed through a gate into the mixer. Here it is given a dry 

 mix for about 15 seconds, and then the correct weighed amount 

 of asphalt cement at about 325° F. is added, and the whole is 

 mixed for at least one minute, or until it is a homogeneous, bright 

 black mass like a blackberry in appearance. By means of a slide 

 gate in the bottom of the mixer it is dropped into a wagon. A 

 sufficient number of batches are dropped in to complete the load, 

 which is then hauled to the street, dumped, spread with shovels, 

 raked to the required thickness, and rolled till too cold to take 

 more compression. This is a brief outline of the construction of 

 the wearing surface of a sheet asphalt pavement. In order to 

 prevent movement on the foundation an intermediate layer, called 

 the binder course, is interposed. This is made from the one inch 

 rock in the other compartment of the sand bin mixed with about 

 25 per cent, of sand, all cemented together with asphalt cement. 

 The increased friction of the stone holds the pavement firmly to 

 the foundation under the shoving effect of traffic. 



Asphalt Concrete is made similarly, being a sheet asphalt 

 mixture with about 30 per cent, of f inch stone added. The 

 addition of the stone will decrease the surface area, and therefore 

 the required amount of A.C. and dust; and is therefore propor- 

 tionately cheaper. It is, however, never laid less than 2 inches 

 thick. 



